This invention relates to the positioning of biomeasurement sensors close to the body of a subject, and, more particularly, to the accurate positioning of a biomagnetometer dewar tail adjacent to the body of a human subject.
The biomagnetometer is a device for measuring small magnetic fluxes that are produced by a living subject. Small electric currents flowing in the body produce magnetic fields, and the biomagnetic fields are detected by input coils connected to detect the biomagnetometer. The biomagnetic fields are typically on the order of one ten-millionth of the magnitude of the earth's magnetic field, requiring the use of very sensitive detectors, special shielding, and sophisticated electronic signal processing.
The most sensitive detector available, a superconducting quantum interference device ("SQUID"), is operated at superconducting temperatures, 10K or less for the most sensitive of such SQUIDs. The SQUIDs must therefore be contained within a device that maintains such low temperatures. In the most commonly used practice today, the pickup coils and SQUIDs are placed into a dewar vessel and cooled with a cryogenic fluid such as liquid helium.
The dewar is supported from a gantry or stand placed adjacent to the subject. The support structure must be sturdy, inasmuch as current-generation biomagnetometer dewars are typically about 4 feet long, 18 inches in maximum diameter, and 200 pounds in weight when loaded with cryogenic fluid.
The support structure must also be readily operable by hospital technicians to position the dewar precisely and accurately at a selected location near the subject. Once the dewar is positioned, it must remain stably locked at that position. It also must be easily and precisely repositioned, as initial measurements may indicate the need to move the instrument only a few centimeters.
The magnetic fields produced by a subject are very small in magnitude, and their intensities decrease rapidly with increasing distance from the region of the body producing the field. The dewars are therefore typically designed with a reduced section, tubular dewar tail extending from the main body of the dewar. The lower surface of the dewar tail is generally shaped to fit the contours of the human head, with specific dimensions selected based upon a statistical survey of head shapes and sizes. The pickup coils and detectors are contained at the bottom of the dewar tail, so that they may be placed very close to the subject's head in an accurate manner. Where the dewar contains multiple pickup coils and detectors, as is usually the case, it is desirable to position the pickup coils in a preselected arrangement with comparable pickup coils equally spaced from the region of the subject being measured. The various pickup coils can therefore detect the magnetic fields most efficiently to produce a mapping of those magnetic fields, from which the characteristics of the sources within the head may be inferred.
Hospital technicians expend a great deal off time in each instance attaining a precise positioning of the biomagnetometer dewar tail. It is difficult to position a tubular dewar tail precisely adjacent to the human head, and then reposition it precisely at another location if that is required by the initial results. The dewar and gantry are counterweighted, but are still difficult to move in a precise manner. Moreover, the patients may be sensitive to contact by the dewar, and care must be taken to avoiding such contact.
A number of different dewar positioning techniques are used or have been proposed. In one, the technician positions and repositions the dewar based upon his Judgment, an approach that lacks reproducibility and precision. In another, various electronic techniques using lasers, proximity sensors, or other sophisticated tools have been proposed. These techniques may be operable, but add significantly to the cost of the biomagnetometer and may be unreliable. Moreover, most types of electronic devices must be specially engineered to remove any trace of a remnant magnetic field after they are turned off, as such fields may interfere with the biomagnetic measurements. In a third approach, conventional positioning methods used for other types of medically related equipment have been tried. In many instances, the positioning techniques seem to work well for positioning a device at a distance of a foot or more from the body, but do not work well when they are used in an attempt to position the dewar reproducibly to within a few tenths of an inch or less of the subject.
There has been proposed no simple, reliable technique for precisely positioning a heavy object such as a dewar tail closely adjacent to a subject. Such a technique would desirably permit quick, reliable, reproducible initial positioning and repositioning as necessary. The present invention fulfills this need, and further provides related advantages.